In a wireless communications network, devices need to transmit information by using a frequency resource. The frequency resource is also referred to as a spectrum. The spectrum may be classified into a licensed spectrum and an unlicensed spectrum. The licensed spectrum is a frequency resource exclusive to some operators. The unlicensed spectrum is a common frequency resource in the wireless communications network. With development of communications technologies, an amount of information transmitted in the wireless communications network gradually increases. Preempting the unlicensed spectrum to transmit information may improve a data throughput in the wireless communications network, and better satisfy a user requirement.
In the prior art, to improve reliability of radio channel transmission, a Hybrid Automatic Repeat Request (HARQ) is widely used as a primary data retransmission method. For example, after a first device sends data to a second device, the second device feeds back an Acknowledgement/Negative Acknowledgement (ACK/NACK) message to the first device, and the first device determines whether to retransmit the data to the second device, according to the feedback information of the second device. If the data is required, the data is retransmitted. Such a data transmission process is an HARQ process.
For example, in a Licensed-Assisted Access Using Long Term Evolution (LAA LTE) system, when a sending device transmits data by using an unlicensed carrier, the sending device cannot always occupy a channel. Therefore, after the sending device occupies the unlicensed carrier for a particular time, the sending device needs to re-preempt a channel. Consequently, retransmission of the HARQ is interrupted and a receive end can receive the data only after the sending device re-preempts a channel. How to implement HARQ retransmission using an unlicensed carrier becomes a problem to be resolved urgently.